Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 511:

  Morphine alkaloids derived from the opium poppy have long been used as analgesics. Codeine, the methyl ether of morphine, is a naturally occurring alkaloid with medicinal properties very similar to those of morphine. Thousands of derivatives of morphine have been synthesized and tested for their biological effects. For example, the diacylated derivative of morphine, heroin, is a highly addictive drug. Much effort has gone into understanding how morphine and its derivatives function.

  

  Studies have shown that certain common structural features of alkaloids are required for the compound to exhibit biological activity. These structural requirements are summarized by the so called "morphine rule":

  Demerol and methadone, shown in Figure 2, are two synthetic alkaloids designed to satisfy the "morphine rule." Synthetic alkaloids such as these have been found to mimic certain physiological properties of morphine and its derivatives, and

  have found pharmacological application due to other, more desirable biological effects. Methadone has been used widely in the United States and Great Britain as a treatment for heroin addiction; it reduces the physical symptoms

  accompanying withdrawal without producing many of the other effects of heroin.

  

  Meperidine (demerol) Figure 2

  One of the requirements of the "morphine rule" is that an aromatic ring be attached to a quaternary carbon in order for the molecule to be biologically active. The quaternary carbon of any morphine-like substance must be:

  I. a stereocenter

  II. sp3 hybridized

  III.

  sp2 hybridized

  A.

  I only

  B.

  II only

  C.

  I and II only

  D.

  I and III only

  Correct Answer: B

  A quaternary carbon is bonded to four alkyl groups. Any carbon that forms four bonds adopts the tetrahedral configuration and must be sp3 hybridized. sp3 hybridization occurs when one s orbital and three p orbitals mix to form four equivalent hybrid orbitals capable of forming four bonds. Choices A and C are incorrect because a quaternary carbon (bonded to 4 alkyl groups) need not be a chiral center if two of these alkyl groups are identical. This is shown below:

  

  Choice D is incorrect because an sp2 hybridized carbon adopts a planar configuration and can only bind to three substituents (not the four required of a quaternary carbon).

• Question 512:

  In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid

  the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two

  hypotheses were proposed to explain these remarkable results:

  Hypothesis 1

  Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide,

  called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below:

  C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N scotophobin

  Hypothesis 2

  Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and

  vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber.

  They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats

  and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1.

  

  Figure 1

  The chemical analysis of Ungar's extract most likely weakened his claim that scotophobin transferred the learned fear of darkness because it showed that:

  A. the extract was not pure scotophobin.

  B. the extract did not contain scotophobin.

  C. scotophobin was not present in sufficient concentrations to illicit a response.

  D. scotophobin was similar in structure to other proteins.

  Correct Answer: A

  The electrophoresis of Ungar's extract shows that multiple compounds were present. This weakens the argument that scotophobin alone is responsible for the apparent transfer of learning. Choice B is incorrect because the extract does have

  a band at a similar displacement as that of the synthetic scotophobin indicating that scotophobin is present. Choice C is incorrect because the passage never discusses the concentration requirements for scotophobin to elicit a response.

  Electrophoresis is useful for identifying the proteins in a mixture, but it is not as useful for calculating concentrations. This information cannot be concluded from the data.

  Choice D is incorrect because the electrophoretic data does not provide information about the structure of the other proteins in the extract.

• Question 513:

  In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid

  the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two

  hypotheses were proposed to explain these remarkable results:

  Hypothesis 1

  Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide,

  called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below:

  C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N Scotophobin

  Hypothesis 2

  Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and

  vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber.

  They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats

  and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1.

  

  Figure 1

  Researchers were interested in purifying a second protein (protein X) from Ungar's extract. The gene segment encoding protein X was believed to consist of thirty nucleotides. According to Figure 1, which band could represent protein X?

  A. Band A

  B. Band B

  C. Band C

  D. Band D

  Correct Answer: D

  If the amino acid sequence encoding for Protein X is thirty nucleotides long, then Protein X consists of 10 amino acids. Protein X is thus smaller than scotophobin. The technique of electrophoresis uses an electrical field to separate proteins

  based on size. Larger proteins subjected to the same electric field will move more slowly than smaller proteins. Band C on Figure 1 which is common to the extract and to the purified protein represents scotophobin. Bands A and B which are

  closer to the top of the gel would represent proteins larger than scotophobin. Band D, which is below band C, represents a protein that has migrated further and is thus smaller than scotophobin. Of the choices, only band D could potentially

  represent Protein X.

  Choices A and B are incorrect because bands A and B, which have migrated less than band C, represent proteins that are larger than scotophobin. Choice C is incorrect because band C is common to both the extract and the purified protein,

  and must therefore represent scotophobin itself.

• Question 514:

  In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid

  the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two

  hypotheses were proposed to explain these remarkable results:

  Hypothesis 1

  Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide,

  called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below:

  C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N Scotophobin

  Hypothesis 2

  Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and

  vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber.

  They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats

  and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1.

  

  Figure 1

  Researchers isolated a polypeptide from the brains of goldfish trained to avoid darkness. This goldfish scotophobin was 15 amino acids long and differed from rat scotophobin by one amino acid. The gene for goldfish scotophobin must differ from that of rat scotophobin by:

  A. one amino acid.

  B. three codons.

  C. at least one nucleotide.

  D. at least three nucleotides.

  Correct Answer: C

  A codon, which codes for a single amino acid in the final protein, consists of three nucleotides. At least one nucleotide must be changed to code for a different amino acid. Note that changing three nucleotides can also lead to a change in a

  single amino acid, but only if these nucleotides are in the same codon. Choice A is incorrect because the gene consists of DNA, not amino acids. Choice B is incorrect because a change in three codons will lead to a change in three amino

  acids, not one.

  Choice D is incorrect because a change in a single nucleotide can cause a codon to code for a different amino acid.

• Question 515:

  In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid

  the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two

  hypotheses were proposed to explain these remarkable results:

  Hypothesis 1

  Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide,

  called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below:

  C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N scotophobin

  Hypothesis 2

  Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and

  vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber.

  They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats

  and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1.

  

  Figure 1 In a follow-up experiment, researchers administered scotophobin to rats in order to examine its physiological effects. Which of the following observations of physiological effects in the rat would provide the best support for Hypothesis 2?

  A. Increase in urine volume

  B. The rat pupils dilate causing the rat to prefer darkness and avoid light

  C. Increase in heart rate

  D. Dilation of arterioles regulating blood flow to the digestive tract

  Correct Answer: C

  Hypothesis 2 states that scotophobin, which is similar to ACTH and ADH, increases sympathetic activity. Thus, any observation of ACTH or ADH-like effects, or of increased sympathetic activity (fight-or-flight) would support Hypothesis 2.

  Only Choice C, increased heart rate, is a result of increased sympathetic activity. Choice A is incorrect because ADH-like activity would tend to decrease urine volume by increasing water reabsorption in the collecting duct of the nephron.

  Choice B is incorrect because, although sympathetic activity will lead to pupil dilation, if this causes the rats to prefer darkness then it does not support Hypothesis 2 which tries to explain why an increase in sympathetic activity would cause

  the rats to avoid darkness.

  Choice D is incorrect because sympathetic activity will tend to decrease blood flow to the digestive tract. Dilating arterioles would increase blood flow. Also, vasopressin acts to constrict blood vessels and increase blood pressure.

• Question 516:

  In 1972, Georges Ungar reported the discovery of a peptide that appeared to transfer learning. Ungar's claim was based on experiments in which rats placed in a chamber with specially designed dark and light regions were trained to avoid

  the dark regions of the chamber. Following their training, the rats were killed and brain extracts were prepared. These brain extracts were injected into naive rats which were then observed to acquire the fear of darkness without training. Two

  hypotheses were proposed to explain these remarkable results:

  Hypothesis 1

  Ungar concluded that the extracts contained some chemical that transmitted the learned fear of darkness to the naive rats. A fifteen amino-acid polypeptide was isolated from the brain extracts and sequenced. Ungar claimed that this peptide,

  called scotophobin, was a chemical transmitter of learning. The peptide had the primary structure shown below:

  C-ser-asp-asn-arg-gln-gln-gly-lys-ser-ala-arg-gln-glygly-tyr-N scotophobin

  Hypothesis 2

  Other researchers, who tested scotophobin but could not reproduce Ungar's results, argued that scotophobin did not transfer the learned fear of darkness. Instead, they suggested that scotophobin, which is structurally similar to ACTH and

  vasopressin, acted to increase stress in the rats. Since stress increases sympathetic nervous activity, rats injected with scotophobin would become hyperactive and tend to spend less time in the dark regions of the experimental chamber.

  They argued that such stress responses in the rats could be misinterpreted as a fear of darkness. Ungar's claim was further weakened by chemical analysis in which both the scotophobin extracts which Ungar had injected into the naive rats

  and a sample of synthesized scotophobin peptide were subjected to SDS polyacrylamide gel electrophoresis, as shown in Figure 1.

  

  Figure 1

  Hydrolytic enzymes cleave polypeptides at specific amino acid residues. Which of the following hydrolytic enzymes could be used to cleave scotophobin into three fragments?

  

  A. Trypsin only

  B. Trypsin or clositripain only

  C. Clositripain or chymotrypsin only

  D. Clositripain or pepsin only

  Correct Answer: D

  In order to cleave scotophobin into three fragments, the protein must be cleaved at two sites. Clositripain will cleave scotophobin at both Arg residues, while pepsin will cleave scotophobin at the Asp residue and at the Tyr residue. Note that

  the enzymes cleave at the carboxy end of the residue. Thus, according to the direction in which the sequence of the protein is written in the text (carboxy to amino end), the enzymes will cleave before, i.e., to the left of, the specified amino

  acid residue. Choice A and B are incorrect because trypsin will cleave the peptide at two Arg location and one Lys location, creating 4 fragments.

  Choice C is incorrect because chymotrypsin will cleave the peptide at a single point (Tyr), creating 2 fragments.

• Question 517:

  Which of the following functional groups are NOT found on the side chains of any of the naturally occurring amino acids?

  A. Hydroxyl

  B. Methyl

  C. Carboxylic acid

  D. Aldehyde

  Correct Answer: D

  Aldehydes do not occur in the 20 amino acids found in naturally occurring proteins. Choice A is incorrect because the hydroxyl group is found on serine and threonine. Choice B is incorrect because the methyl group is found on alanine and a

  number of other nonpolar amino acids.

  Choice C is incorrect because the carboxylic acid functionality is found on the side chain of aspartic acid and glutamic acid.

• Question 518:

  When humans are submerged in water, the mammalian dive reflex acts to alter circulation. Heart rate decreases, blood flow to the extremities is reduced, and mean arterial blood pressure is increased. These accomodations lead to:

  A. decreased oxygen demand by the tissues.

  B. increased heat retention by the body.

  C. decreased partial pressure of oxygen in the blood.

  D. increased venous return.

  Correct Answer: B

  The question stem describes the effects of the mammalian dive reflex, indicating that blood flow to the extremities is reduced. This would increase heat retention of the body. Choice A is incorrect because oxygen demand by the tissues

  depends on the metabolic rate of the tissues, which is not affected by changes in blood pressure, heart rate, or blood flow to the extermities. Choice C is incorrect because the dive reflex will not lead to a decrease in oxygen partial pressure.

  The partial pressure of oxygen will decrease during a dive because the individual is holding their breath, not because of the accomodations of the dive reflex.

  Choice D is incorrect because venous return, the amount of blood returned to the heart by the venous circulation, will not be increased. If stroke volume (the volume of blood pumped with each beat) remains constant, the decrease in heart

  rate caused by the dive reflex would lead to a decrease in venous return.

• Question 519:

  Which of the following molecules would produce the NMR spectrum shown below?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  The NMR spectra shows a distinct absorption split into 7 peaks, the classic absorption of an isopropyl group. This absorption is downfield, indicating that it is deshielded. From this information alone, we can determine that there is at least one proton with 6 equivalent neighboring protons, that is near some electronegative species. Only choice A has 6 neighboring protons that can lead to splitting into (N+1) seven peaks. The downfield shift is caused by the bromine bonded to the same carbon as the single hydrogen.

  

  Choice A is incorrect because there are no neighboring protons, and so there will be no splitting. Choice B is incorrect because there are no neighboring protons and so, splitting into seven peaklets will not occur.

  Choice C is incorrect because the single proton has only 3 neighboring protons and will only be split into 4 peaklets (a quadruplet). Note that the absorption for the single proton in this molecule will be shifted more downfield because there are

  many electronegative species around it.

• Question 520:

  Which of the following best illustrates the contracted state of the sarcomere shown below?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The sarcomere illustrated in the question stem consists of thin and thick filaments. Contraction occurs through the sliding of the thin filaments along the thick filaments, towards the center of the sarcomere. During this contraction, the length of both the thick and thin filaments remains unchanged. This sliding can be seen in the increased overlap between thick and thin filaments. Choice A is incorrect because it indicates that the thick filaments have shortened. Choice B is incorrect because it indicates that the thin filaments have shortened. Choice D is incorrect because it indicates that both the thick and thin filaments have shortened.